1. Field of the Invention
The present invention relates to a titanium dioxide scouring media. Generally, the invention relates to substantially round and spherical sintered titanium dioxide pellets, and the method used involves pelletizing titanium dioxide dust and sintering the green pellet to form the scouring media.
2. Description of the Related Art
The production of titanium dioxide through the vapor phase oxidation of titanium tetrachloride has become a commonly used process in the production of titanium dioxide pigment. During this process, titanium tetrachloride vapors react with oxygen or air or both at high temperatures in an oxidation reactor. The resultant stream of hot oxidation products exiting the reactor contains titanium dioxide in a finely distributed form suspended in gaseous constituents. The freshly formed titanium dioxide particles, which are present at first as very reactive particles, grow together to form small aggregates of sizes below 0.15 mm. Some of these small aggregates grow larger in size and partly sinter to form larger, hard and unshapely aggregates above 0.15 mm in size. These larger aggregates tend to form in a layer and deposit on the surfaces of the hot reaction zone of the oxidation reactor. Continued use of the oxidation reactor and the hot reaction products which pass over it constantly regenerate this layer. Not only does this layer regenerate with continued use of the reactor, but often pieces of this layer break off and are carried away in the reaction mixture. These large particles are not suitable for pigment production and serve to contaminate the reaction mixture. Different processes have attempted to reduce the formation of these unwanted particles but none have been completely successful.
To prevent the formation of this layer of titanium dioxide particles, scouring media consisting of chemically inert scrubbing solids have been added to the reaction mixture at appropriate locations. Numerous substances have been used as scouring media to include aluminum oxide, zirconium silicate, and silica sand. One of the most commonly used scouring media is silica sand. However, there are disadvantages to the use of silica sand as a scouring media in an oxidation reactor.
Silica sand is somewhat angular in shape. This angular shape is effective in scouring the walls of the reactor, but it also severely wears the inner walls of the reactor. Eventually, the silica sand generates holes in the reactor which require repairs or in some cases replacement. The replacement time could be a matter of weeks, thus severely hampering production. Furthermore, these silica sand particles crush easily. The crushed silica sand contaminates the reactor mixture and must be removed from the mixture. If not removed, the silica sand will reduce the quality of the resultant product. Therefore, there exists a need for a scouring media that will be strong and dense enough to effectively scour the titanium dioxide reactor walls but is round and spherical and less abrasive to reduce or eliminate reactor wear. There is also a need for a scouring media that is sized properly to provide adequate separation and recycle and would reduce the potential for contamination of the reactor mixture.
Titanium dioxide has been used as a scouring media to address the disadvantages to using commonly known scouring media. However, the titanium dioxide previously produced for use as scouring media has not eliminated all of the problems associated with other scouring media. In the process described in U.S. Pat. No. 4,784,841, titanium dioxide scouring particles are produced by vapor phase oxidation of titanium tetrachloride. The particle aggregates initially formed which are above 0.15 mm are separated, admixed with alkaline hydroxides, calcined and returned to the process. This process produces particle aggregates of sizes between 0.2 to 2.0 mm for use as scouring aggregates for the prevention of deposit formation on the heat exchanger. This process fails to control the sizes and shape of the aggregates formed, thus failing to reduce the possibility of damage to the reactor walls and failing to maximize separation and recycle/reuse efficiency.
In the process described in U.S. Pat. No. 2,721,626, course scouring solids of sizes between 0.15 and 6.35 mm are admixed with a hot reaction mixture and after cooling separated from the reaction products and used again as scouring solids. The dry separation of the course scouring titanium dioxide particles from the finely particulated titanium dioxide is inefficient. Thus, there is always a percentage of course scouring solids in with the final product thereby reducing the overall efficiency of the process.